Unaudited Quarterly Financial Data (Details) - USD ($) $ / shares in Units, $ in Thousands	3 Months Ended								12 Months Ended
	Dec. 31, 2020	Sep. 30, 2020	Jun. 30, 2020	Mar. 31, 2020	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Quarterly Financial Information Disclosure [Abstract]
Net premiums earned	$ 261,367	$ 256,113	$ 243,562	$ 260,901	$ 266,267	$ 267,857	$ 247,102	$ 249,762	$ 1,021,943	$ 1,030,988	$ 975,162
Investment income, net of expenses	36,118	37,252	39,679	41,347	41,322	42,715	42,423	40,585	154,396	167,045	141,331
Realized gains (losses)	2,901	2,259	6,701	1,891	1,320	4,205	307	(526)	13,752	5,306	(1,353)
Other revenue	1,895	380	4,026	2,754	2,717	3,606	2,485	1,830	9,055	10,638	8,708
Loss incurred, net	45,758	40,686	217,374	60,956	23,690	33,985	21,836	39,064	364,774	118,575	36,562
Underwriting and other expenses, net	66,311	64,253	60,111	57,698	65,227	61,278	59,270	61,650	248,373	247,425
Loss on debt extinguishment	0	26,736	0	0					26,736	0	0
Provision for income tax	38,782	33,518	2,436	38,434	45,599	46,186	43,433	38,996	113,170	174,214	174,053
Net income	$ 151,430	$ 130,811	$ 14,047	$ 149,805	$ 177,110	$ 176,934	$ 167,778	$ 151,941	$ 446,093	$ 673,763	$ 670,097
Earnings per Share [Abstract]
Basic (in dollars per share)	$ 0.45	$ 0.39	$ 0.04	$ 0.44	$ 0.51	$ 0.50	$ 0.47	$ 0.43	$ 1.31	$ 1.91	$ 1.83
Diluted (in dollars per share)	$ 0.44	$ 0.38	$ 0.04	$ 0.42	$ 0.49	$ 0.49	$ 0.46	$ 0.42	$ 1.29	$ 1.85	$ 1.78